How sound is used in thrillers to create tension?

The sound in a film is a valuable element, it can add and take away from a scene. Tension can be built or emotion with the right sound, David Lynch says 'Sound is 50 per cent of a film, at least. In some scenes, it's almost 100 per cent'. 

Two types of sound
Diegetic sound is all the sound that happens there and then, it is not added in later in the editing stages of production. For example, this could be the phone ringing or people talking during the scene.
Non-diegetic sound is the complete opposite of this, it is the sound that has been added in when editing. For example, this could be music being played in the background or a voice on the other end of the phone line. This is probably the easiest and the best way to build tension, for example, the way the background music starts off at a slow pace and then in movement with the action happening on screen the pace slowly starts to become quicker thus making the tension high. 

Examples of sounds being used in Thrillers
Psycho by Alfred Hitchcock in 1960
Known in the film industry as the master of suspense, it is fair to say the master has earned this name with each of his horror and thriller films. In 'Pyscho' he succeeds in this again by prying human emotions together for the elements of sound.
Hitchcock contrasts what the audience sees on screen with the sound playing, the black and white  and simple music juxtaposes the complex psyche.The single tone score of an all string orachesta reflects the classic film noir inspired blacn and white images that feature on screen pinched tone of the film. Hitchcock is a master of capturing tension in the slower portions of the scene, for example this is done when Marion, the main character, steals a large amount of money from the client at her work. Fast paced when Marion is in the car witht the large amount of money sat in the passenger seat, the speed of the soud could be reflecting the pase of her mind and that worrying thoughts are racing around her mind. The violin sound creates a circle of nerves flurrying around Marion, this also creates a sesne of butterflies in her stomach as she is nervous.  Any time Marion handles the large sum of money this style of sound reappears which reinforces the her nervous state because she has stolen the money and clearly feels guilty. 

In slower scenes of the film, Hitchcock retains tension again well throguh ostintato ( a technique in which each notes has the same stress as the last), showing he is the true master of holding tension that he is able to do it in slow and fast scenes of the films. Hitchcock gave birth to the everfamous violin sound which is now incredibly common in horror and thriller films. This violin sounds creates an effcet of eardrums riniging, he uses these violin sounds in the most intense scenes. 
The first example is the infamous shower scene in which Marios is taking a shower in the her motel room.The scene starts out quiet, which creates a sense of vulnerability in which the audience can relate to, also because this scene is incredibly quiet any sudden sounds will make the audience jump. As the murderer approaches Marion the music still doesn't start until the shower curtain is drawn and the knife is drawn which shows the 'mothers' evil plot. With Marion screams a series of violin sounds is heard and the audience ca hear the sound of the knife being dragged through her flesh, which was reportedly recreated with a knife being slashed through a watermelon. The shrieking of the violence may resemble the high pitched shrill of a bird which may suggest to Norman as he admitted to his strange hobby of stuffing dead birds. 

Not only Marion is a victim of Norman's strange and twisted personality, Arbogast (the P.I) also is a victim of Norman, as he approaches the he is supported by the dry sounds of a cello however the music changes to a high pitched sound when the detective enters the house which cnveys that it is a incrediby dangerous sitaution he has now walked into, it may also suggest the nerves he is feeling when he walks in. The music returns to familiar violin screeching which was previously heard with Marion. The violins return encore with the P.I's scream, this shows that when the violins are heard that violence is right around the corner for the character shown on screen. 

Psycho shows how well Hitchcock understands the importance of sound. How suspence is held relies massivley on two facets, cinematic techniques and sound stratergies, by using both of these Hitchcock can capture and manipulate the audience emotions, this again proves his mastery of creating tension through the use of sound.